This invention relates generally to artificial disc replacements (ADRs) and, more particularly, to ADRs facilitating more natural kinematics.
Many spinal conditions, including degenerative disc disease, can be treated by spinal fusion or through artificial disc replacement (ADR). ADR has several advantages over spinal fusion. The most important advantage of ADR is the preservation of spinal motion. Spinal fusion eliminates motion across the fused segments of the spine. Consequently, the discs adjacent to the fused level are subjected to increased stress. The increased stress increases the changes of future surgery to treat the degeneration of the discs adjacent to the fusion. However, motion through an ADR also allows motion through the facet joints. Motion across arthritic facet joints could lead to pain following ADR. Some surgeons believe patients with degenerative disease and arthritis of the facet joints are not candidates for ADR.
Current ADR designs do not attempt to limit the pressure across the facet joints or facet joint motion. Indeed, prior art ADRs generally do not restrict motion. For example, some ADR designs place bags of hydrogel into the disc space which do not limit motion in any direction. In fact, ADRs of this kind may not, by themselves, provide sufficient distraction across the disc space. ADR designs with metal plates and polyethylene spacers may restrict translation but they do not limit the other motions mentioned above. The articular surface of the poly spacer is generally convex in all directions. Some ADR designs limit motion translation by attaching the ADR halves at a hinge.
One of the most important features of an artificial disc replacement (ADR) is its ability to replicate the kinematics of a natural disc. ADRs that replicate the kinematics of a normal disc are less likely to transfer additional forces above and below the replaced disc. In addition, ADRs with natural kinematics are less likely to stress the facet joints and the annulus fibrosus (AF) at the level of the disc replacement. Replicating the movements of the natural disc also decreases the risk of separation of the ADR from the vertebrae above and below the ADR.
The kinematics of ADRs are governed by the range of motion (ROM), the location of the center of rotation (COR) and the presence (or absence) of a variable center of rotation (VCOR). Generally ROM is limited by the facet joints and the AF. A natural disc has a VCOR, that is, the COR varies as the spine bends forward (flexion) and backward (extension). Typically, the vertebra above a natural disc translates forward 1-2 mm as the spine is flexed.
Prior art total disc replacements (TDR), that is, ADRs with rigid plates that attach to the vertebrae, do not replicate the kinematics of the natural disc. Generally, the COR lies too anterior. Most prior art TDRs also rely on a single, fixed COR. As a result, many of the prior art TDRs have a limited ROM.
This invention improves upon prior art TDRs by more closely replicating the kinematics of a natural disc. The preferred embodiments feature two or more fixed centers of rotation (CORs) and an optional variable COR (VCOR) as the ADR translates from a fixed posterior COR to a more anterior COR.
The multiple CORs permit a TDR with a posterior COR that lies posterior to the COR of the TDR to facilitate normal disc motion. The use of two or more CORs allow more flexion and more extension than permitted by the facet joints and the AF. Artificial facet joint-like components may also be incorporated into the design to restrict excessive translation, rotation, and/or lateral bending.